Stace gives other differences but I get that impression that these are not constant. Silene uniflora tends to have larger and few flowers on a stem, and to have narrower stem leaves, and to have a less constricted calyx apex, and to have a procumbent habit. Silene vulgaris tends to be taller, and to be hairier, but both can be glabrous (and I think of Silene vulgaris as a glabrous plant).

Stace also says that hybrids occur where the two species meet.

Silene uniflora has a similar distribution to Armeria maritima - the coast, Scottish mountains, gardens, and casual escapes and throwouts from gardens.

As the other chap says, Silene latifolia has a herbaceous ribbed calyx, whilst Silene vulgaris has a prominently inflated, smooth, calyx, with a porcellaneous appearance. Once you've seen Silene vulgaris the distinction is obvious.

..end of the day are you agreeing with S.latifolila or not? - I do appreciate the low down on Silene - is Stace the book/online definitive plant reference that we should be using and where do I get it from?

Your plant is S. latifolia. Have a look at some of the photos of S. vulgaris - I posted one a few days ago at http://www.ispot.org.uk/node/339349. You can see the different pattern of veins on the inflated bladder.
Stace (3rd edition) is available from Summerfield Books or Amazon, it's the one with all the up to date names, but few pictures.

There's also Sell and Murrell, which is published in a series of volumes at about £150 each. I don't personally know any individuals who have invested in this! Stace is very good for keys and descriptions, and much more affordable at £40-£50. Too heavy to use in the field, though.

... are published (I think that it's 3 of 5), with (I hear) new classifications of Ranunculus auricomus and Polygonum promised. I'm tempted to get the Campanulid? volume (Campanulaceae to Asteraceae) for hawkweed and dandelion identification, but I guess that until I learn to identify them to section this would be premature.

... - even though the 3D ribbing of the calyx isn't obvious on this 2D photograph, the calyx of Silene vulgaris is white with reticulate pink veining.

The 3rd edn of Stace is now the "standard" British Flora. (Sell and Murrell goes into greater detail, especially in "critical" groups such as brambles, hawkweeds and dandelions, and in regards to infraspecific taxa, but not all volumes are published yet.) There is no such thing as a definitive reference - botanists are never united on all points, and scientific knowledge continues to march on - but Stace is the nearest that you'll get.

Stace is more comprehensive than any other contemporary flora, including ferns and allies, conifers and other trees, and grasses, sedges and rushes. However specialist tree guides describe more cultivated taxa.

Stace has no photographs and few illustration (all in black and white). To use Stace you need to be able to identify plants from keys and textual descriptions.

I like Sterry (Collins) and Streeter, which combine photographs and illustrations respectively with field marks, and use Stace for the more difficult taxa. (Streeter is too bulky to use in the field - but so is Stace.)

A lot of people like Rose, which offers keys and illustrations.

Keble-Martin is a bit dated, but does provide illustrations of all halfway common sedges and grasses. (Sterry only provides the commonest sedges and grasses, and most general field guides omit them altogether.) Consequently it was Keble-Martin that let me crack most of the common grasses (I'm still got to crack Poa, Agrostis, Schedonorus and Festuca.)

I bought the 1st edition of Stace from a local bookshop, and the 3rd edition from Amazon. (I forget whether it was from Amazon itself, or from Amazon Marketplace.) If you search for "stace new flora of the british isles" you'll find lots of places selling it (ignore the place selling a 2nd edn for £500 - that's ridiculous). You can also order it through most booksellers, and might find copies in larger and university bookstores. Major libraries are also likely to have copies.

You can see a bit of the book at Amazon (Look Inside), and rather more at Google Books.

If you just want a definitive list of taxa I think that you can get that from the BSBI web site (www.bsbi.org.uk).

According to the Wildflower finder website, the main difference is that White Campion has ridges on the enveloping calyx (sepals) whereas Bladder Campion doesn't and instead is inflated and has a net pattern. I hope this helps.

See http://wildflowerfinder.org.uk/Flowers/C/Campion(White)/Campion(White).htm